Hey All! I have a computrainer, and I just purchased a Powertap. After having the PT installed, I figured I’d put the bike on the computrainer, to see how close the wattage readings were between the computrainer and the PT. Well, they weren’t even close! My computrainer was reading about 210 watts while the PT was reading more than 325, steady. Now, unless I’m about to enter the Tour de France, there’s no way I’m putting out 300+ watts for 1.5 hrs. Has anyone had this problem, and is this a calibration issue with the PT or Computrainer? Any ideas how to fix it???
I’ve only just taken possession of my CT and noticed that at threshold HR’s my wattage seemed to be about 10watts lower than when tested in lab. It’s not a specially calibrated version but I’ll be curious to see how comparable it is as I use it a bit more.
Just on CT’s, why do so many people say they are the best training tool you can get when to be honest you can do the same workouts on a normal trainer. Granted you don’t have access to spinscan, wattages etc but in terms of effort you can get the same from the cheapy can’t you? Just started doing my planned trainer sessions on the CT and wondered how it would make me so much better?
Just on spinscan, do the numbers really mean that much. I’ve found I can average mid 70’s but don’t know whether that’s considered bad, ok, or good. It’s impossible to get higher figures when spinning easy (for me anyway!).
assuming that you zeroed or made sure the torque was zero prior to your comparison, it’s most likely that your PT is correct. it’s very rare for a PT to be incorrect (assuming that it’s been zeroed). occasionally, the power goes wrong – normally when the torque tube fails, however, they seem to last for several years before that happens (at least in my experience). once the torque tube fails, they need to be sent back to cycleops to be repaired.
as to whether or not you’d be good enough for the TdF, this will depend on many factors, not least your height and mass, and whether the 300+ W was an easy ride or not.
ric
While you maybe able to get the same workout on a normal trainer you can’t track much but cadence and speed & distance (if you have a rear wheel speed pick up). For people like myself, who enjoy’s being able to analyze my workout date, and chart improvement in my fitness the CT is invaluable. I’ve done about 80% of my riding on the CT since last August and expect it to pay off considerably this race season. Also, the spinscan, allows you to really see when your body becomes fatigued…
Regarding spin scan numbers… My readings/research suggest you should attempt to keep your spin scan numbers as high as you can. 100 is perfect, while no person is perfect it’s what you should strive for. I’ve read that triathletes and roadies should maintain between 80+. I personally picked up my CT last August. When I first started I was in the mid 70 and couldn’t hold anything above 70 when in high gears. About 1500+ miles later on the CT, I can easily maintain 85+ spinscan numbers in a 39/17 for an hour. After an hour my hip flexors really become fatigued and I end up slowly “mashing” with numbers in the 70’s.
Still don’t see how those wattage #'s are correct (325 for 1.5 hrs), and the computrainer is registering right around 210. I’m an average cyclist I’d say, with a 5:40 bike split at IMFL last year. It also looked like the speedometer was reading at a much faster MPH than the computrainer. It was registering close to 30 MPH. NO WAY I can bike that fast…
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Did you zero the torque?
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Did you accurately measure and input your wheel size?
I’ve had and consistently used a PT for four years, with nary a problem like this.
As another New Jersey triathlete, tell us your name, and what races are you doing, so we, too can see how well CT training and SpinScan work.
Just on CT’s, why do so many people say they are the best training tool you can get when to be honest you can do the same workouts on a normal trainer.
On a normal trainer you can back off your pace. If you get the coaching software and are doing 2x20 workouts the wattage stays put regardless of how tired you are. There is no slacking off. That is the biggest advantage I see in my usage of the CT.
t
What kind of rear wheel were you using? I’m certain that this is a demonstration of the amount of power you can loose in a spoked rear wheel, since PT measures at the hub. What you need to do is try it again at the same heartrate and RPE with a rear ZIPP disk and a conti GP, and the CT will now be at 300W, showing again the benefits of a good stiff disk and smooth tire, easily worth 100W (or about 1 hour at an ironman). If you added an SRM into the mix, you would probably be reading about 375, demonstrating that you need a stiffer frame and C-Record components to eliminate all these drivetrain losses. Your next IMFL split should be about 4:10 I predict, thank me later.
Actually, in the first post you say the CT was higher, in your second you say it is the PT. Now I think you are just making this up.
LOL! I’ve got a Velocity Rim on it, so I guess it really is the Equipment:) Seriously, it may be that I just haven’t zeroed out the torque. Typical male, don’t read the directions:) I’ll check that out. I’ve never averaged 300+ on a Computrainer, not even close…If I ride steady pace, I’m usually around 200-210, NOT the 325 the PT was telling me. I think I would drop dead from Cardiac arrest before I could put up those kind of watts. I’ll check it out. Man, I’m hoping for 5:30 or so bike split at IMFL this year. You mean to tell me if I get all the right gear, I could break 4:30??? Sign me up:)
Well, you could do like I do…with my running and swimming skills a 4:30 bike still wouldn’t put me in the money, so I just “assume” I could bike 4:30 if I spent the money, and I adjust my final finish time by that amount. With my engineering degree and a computer I can usually do the math in 15 minutes or so 
The results from one another are not comparable. You can only compare results from the same unit to the same unit. Two different Computrainers will give slightly different readings too. Different types of apparatus give wildly different readings.
So what you are implying is that neither unit is right and you should simply use one unit to set a baseline and then try to up the rests off of that baseline. I could do that for a lot less money. And I thought parts is parts and watts is watts. Shows you what I know.
My advice. Get your money back. From both. Then listen to Dan, he seems to have it dialed.
From bryanjaf, "So what you are implying is that neither unit is right and you should simply use one unit to set a baseline and then try to up the rests off of that baseline. I could do that for a lot less money. And I thought parts is parts and watts is watts. Shows you what I know. "
I don’t think this is what was implied and I certainly don’t believe it is true. I know that with my Powertap, I can hang a weight from the pedal with the crank horizontal and the wheel fixed and get a torque reading which is accurate and reproducible. I have also done this with other’s units and have found the same results. I know people have done the same procedure on SRM’s and found them accurate and reproducible, as well. So for these two systems I’m familiar with, I would say that either will produce a reading which is “right”.*
*realizing that the SRM and Powertap will differ by the losses in the drivetrain as they are measuring power at different points. Data taken on bikes equipped with both units have shown that the readings are essentially equal after taking these losses into account.
they should not give you wildly different readings in the accuracy dept. even if they are reading power at different places. that’s all i am saying. you took my post to literally, which is my bad.
bryanajaf wrote, “hey should not give you wildly different readings in the accuracy dept. even if they are reading power at different places. that’s all i am saying. you took my post to literally, which is my bad.”
I’m not sure which “they” you’re referring to. Pwertap and SRM are both accurate in the hanging weight test. In riding conditions, Powertap and SRM differ by 2-4% which is a reflection of the drivetrain losses. There have been discussions about which is the most relevant measure. The SRM gives a closer indication of the stress on the rider, the powertap gives a better measure of the power propelling the bike for use in models such as analyticcycling.com. In either case, the data is close of enough to be used to guide training. The computrainer is a different matter. I’m no expert on that system, but my understanding is the absolute values from the CT may agree with the other systems but in many cases does not.
Tom stated that it is his experience that different apparatus for measuring power (I believe he was implying CT and PT) could give “wildly different readings”.
My point was merely that power should be power and devices that measure it should give you the same reading at the same power level within a tolerance. The fact that a CT and PT, or SRM for that matter, can give you “wildly different readings” is concerning given that these things are costly training devices. I would not want to sink a g into a training tool if it was not giving me accurate readings. End of story.
Making sure my PT is zeroed before using it, and setting the press-on force on the CT to 2.0 (as close as possible to that exact number, within 0.1), I have found my PT and my CT to give me nearly identical readings.
The fact that a CT and PT, or SRM for that matter, can give you “wildly different readings” is concerning given that these things are costly training devices. I would not want to sink a g into a training tool if it was not giving me accurate readings. End of story.
All indications are that a properly calibrated PT or SRM give consistent, accurate power readings. A single Computrainer may be consistent, but it is not accurate, nor comparable to another Computrainer. The only “wildly different readings” among these is the CT. If you want accurate, get a PT or SRM, not a CT. Real end of story.
klehner wrote, "All indications are that a properly calibrated PT or SRM give consistent, accurate power readings. A single Computrainer may be consistent, but it is not accurate, nor comparable to another Computrainer. "
I agree, but would change “is not accurate” to “may not be accurate”. My experience (as seconded by CarolJ) is that my computrainer agrees within a couple of percent of the powertap.